In order to deal with an increase in Internet traffic in recent years, introduction of digital coherent communication to long distance optical communication has been initiated whereby an increase in capacity of the long distance communication is in progress. Furthermore, application of the digital coherent communication to medium to short distance optical communication is also under consideration. In particular, an optical transceiver for use in the medium to short distance optical communication faces a limitation on an installation space available. According to the CFP2 standard under consideration for the introduction to the medium to short distance optical communication, components of the optical transceiver need to be put into the size of about 80 mm×40 mm. In addition, even smaller standards such as the CFP4 standard are also under consideration. The optical transceiver requires significant reduction in size from the standard in a range of about 130 mm×100 mm to about 180 mm×130 mm conventionally used in the long distance optical communication. To this end, there is a strong demand for reduction in size of an optical modulator which is a component used in the optical transceiver.
As such optical modulators in the optical transceivers for the digital coherent communication, LN (lithium niobate) optical modulators have heretofore been widely used. However, it is difficult to reduce the size of such a LN optical modulator. Hence, the use of a semiconductor optical modulator applying semiconductor such as InP (indium phosphate), which enables further reduction in size, has been devised. Non Patent Document 1 describes a configuration of the semiconductor optical modulator using InP.